Carbonate reservoirs introduce great challenges due to their complex fabric nature (low matrix permeability, poor effective porosity, fractures) and unfavorable wettability. These challenges are further displayed when combined with increased depth and low grade oil (low API and high viscosity). A huge amount of oil is contained in such reservoirs without any technological breakthrough for improving the recovery efficiently.
The main recovery mechanism in fractured carbonate reservoirs is matrix-fracture interaction. The most proven approach to produce heavy-oil reservoirs is through thermal means, specifically speaking steam injection. Yet, the typical reservoir engineering approach is based on mobility increase by reducing oil viscosity through effective heating, and by producing oil through viscous and gravity displacement. This is valid in homogeneous sandstones. Carbonate systems, which are fractured in general, introduce rock complexity at different scales, i.e., faults, fissures, micro fractures, vugs, poorly interconnected matrix pore structure, etc. Wettability is also a very important feature which controls the location, flow and distribution of fluids in the reservoir. When these two effects, i.e., inhomogeneous rock and unfavorable wettability, are combined with high oil viscosity, oil recovery from this type of reservoir becomes a real challenge and classic thermal application theories fail to define the displacement process.
Oil recovery from fractured carbonates relies on drainage of matrix where a great portion of oil is stored. Wettability is a critical factor controlling this drainage process in both immiscible (water or steam flooding) and miscible (solvent injection) displacement. It is essential to have a water-wet medium to drain matrix oil in fractured carbonates in immiscible processes. Carbonates, however, usually fail to meet this criterion and therefore are not eligible for this type of application. Alteration of wettability from oil-wet to water-wet may introduce technical and theoretical challenges if not well understood for specific cases. If wettability alteration occurs, it will occur mostly near the fracture and progress through the matrix as the elevated temperature front progresses through the matrix.
If waterflooding is not responding due to unfavorable wettability and low gravity of oil, recovery can be improved by reducing oil viscosity to enhance matrix drainage. As the matrix is still not water-wet enough to cause recovery by capillary imbibition, gravity is expected to be the governing force to drain oil. Thermal Assisted Gas Oil Gravity Drainage process (TA-GOGD) provides a glimpse of hope on getting better recovery by reducing matrix oil recovery. However, the project life is still long. Operationally, such recovery techniques are totally water dependent. The challenges are then not due to water injection/production only, but also on water availability and disposal. Yet, the oil recoveries are below the economical limit as the drainage is a slow process and the ultimate recovery from the matrix is expected to be relatively low.
Although part of the water may be treated and re-injected as steam, water treatment to insure 0 ppm of oil is expensive and risky for water boilers.
These theoretical and operational challenges urge for a different approach in tackling heavy-oil recovery from fractured carbonates.